This invention relates to polycrystalline metal oxide varistors. More particularly, this invention relates to a novel configuration of polycrystalline metal oxide varistors by which the breakdown voltage of the varistor is made to occur at a lower voltage. The term "breakdown" is not meant to denote device failure, but is used only to designate a value of voltage across the device beyond which the current through the device increases dramatically. That is to say, for voltage values below the breakdown voltage, the device behaves like an ohmic resistor of very large value (in the megohm range) but when the breakdown voltage is exceeded, the device behavior is very much like that of a low resistance conductor. These devices exhibit a very nonlinear current voltage characteristic which is expressed by the equation ##EQU1## where I is the current flowing through the material,
V is the voltage across the material, PA1 C is a constant which is a function of the physical dimensions of the body of the device, its composition, and the parameters of the process employed to form the body, and is a measure of the voltage at which breakdown occurs, and PA1 .alpha. is a constant for a given range of current and is a measure of the nonlinearity of the resistance characteristic of the body.
Metal oxide varistors are sintered ceramics composed principally of zinc oxide with a mixture of various other metal oxides added. These other oxides are typically bismuth trioxide, cobalt trioxide, manganese dioxide, antimony trioxide, and tin dioxide, each being present to the extent of approximately one-half to one mole percent, the remainder of the material being zinc oxide. This powder is ground and pressed into the desired shape after which the material is sintered at a temperature of approximately 1,000.degree. C. to 1,400.degree. C. After this, electrodes are applied to faces of the material. Wires are then attached to the electrode surfaces for connection to external circuits.
The materials and processes for making metal oxide varistors are well known in the art and are described, for example, in U.S. Pat. No. 3,962,144 issued to Matsuura et al.
The metal oxide varistor, in many respects, is similar to a bidirectional Zener diode, in that when a voltage is applied in excess of a certain value, the characteristic resistance of the device decreases dramatically and conduction through the device occurs. This non-linear conduction characteristic renders the metal oxide varistor an important element in a variety of protective circuit configurations. The better metal oxide varistors have a value of alpha in excess of 10. In general, the higher the value of alpha the sharper is the transition from the non-conductive to the conductive operating regions of the device.
One of the deficient characteristics of low voltage varistors manufactured in accordance with the prior art is that they have a high upturn value. That is to say, the value of the exponent alpha becomes lower at higher current values. This means that the voltage across the device becomes too high too quickly. It is thus said that a varistor should have low upturn.
Similarly, when the varistor is operating below its breakdown voltage, it should exhibit a high resistance characteristic. This means that for a given voltage the current through the device should be minimal. A typical voltage point that is chosen is one-half of the breakdown voltage and at this voltage the current through the device is measured and is referred to as the leakage current. Hence, it is desired that varistor devices have a low value for this leakage current.
The process of making varistors described above crystals of zinc oxide in the material. These crystals range in size from between 5 and 50 microns, the typical grain size being approximately 25 microns. The varistor effect that is produced occurs as a result of grain boundaries between the zinc oxide grains; and it is known that each grain boundary exhibits a breakdown voltage of approximately 3 volts. Hence, a one millimeter slab of the sintered ceramic metal oxide varistor powder possesses approximately 40 grain boundaries on the average, assuming a grain size of 25 microns. This results in a breakdown voltage of 120 volts. Hence, in order to get varistors where the cut-off voltage lies in the range below approximately 40 volts, it is necessary to produce ceramic slabs of varistor material well below one-half millimeter in thickness. Slabs of this thickness have extremely poor mechanical properties and fracture very easily.
It thus appears that in order to get low breakdown voltage varistors, one can have varistor material of extreme thinness. One of the solutions posed for this problem is described in Japanese open patent No. 51-52988 (1976), in the name of Satoru Ogihara et al. The Ogihara et al. patent appears to disclose a procedure in which bismuth and manganese oxide are mixed with zinc oxide in a powder form and heated at 1,000.degree.-1,400.degree. C. in an oxidizing atmosphere, after which the powder is mixed with an organic or inorganic binder and following this step the resultant mixture is pressed or otherwise shaped into the desired form. Ogihara et al. appears to disclose that this material may be pressed into thin sheets whereby the breakdown voltage may be lowered. However, these devices lack mechanical strength.
However, at present, it is difficult to get quality varistors where the breakdown voltage lies in the range below approximately 40 volts. These devices have high leakage current, a mediocre value of the exponent alpha, high upturn, and poor operational stability compared to varistors operating at higher breakdown voltages. One of the other methods for solution to this problem has attempted to increase the size of the zinc oxide grains and therefore to reduce the number of zinc oxide grain boundaries. Typically, this is done by the addition of grain growth accelerators such as titanium dioxide. However, this method of lowering the breakdown voltage has the disadvantage of causing current channeling through the varistor due to local exaggerated grain growth, thereby causing nonuniform current conduction across the surface of the device.
Another method for producing low breakdown voltage varistor devices is by decreasing the over-all thickness for a given grain size. This method results in a device with poor mechanical properties, namely, such devices are easily fractured in handling and therefore it becomes economically unattractive to manufacture devices of such small thickness by the usual automated methods. In the configuration in which polycrystalline metal oxide varistors are presently fabricated, the varistor material is pressed into a disk shaped body having a pair of opposed major faces and the disk is then sintered to produce a ceramic varistor disk. An electrode coating is applied to the opposing major face ot each disk, most commonly by applying a silver electrode paste which is then fired onto the ceramic disk. Other commonly used methods include metal evaporation, plasma spraying, or coating with an indium-gallium eutectic. Wires are then attached and the device is encapsulated in an appropriate package.
Lowering the breakdown voltage on these devices by increasing the zinc oxide grain size is not an attractive alternative. This method leads to current channeling through the disk in which very small cross-sectional areas of the disk carry almost all of the current flowing through the disk causing thermal hot spots within the disk which may cause it to fracture or otherwise contribute to premature failure of the disk.